There are numerous examples of diffraction-based laser Doppler velocimeters that are used to measure the length of a moving surface. Some of these devices use diffraction gratings to split a laser beam into two beams and then recombine them to establish an interference pattern in the vicinity of the moving surface to be measured. Some examples are described in publication by J. Schmidt et al, Diffractive Beam Splitter for Laser Doppler Velocimetry Optics Letters/Vol 17, No 17/Sep. 1, 1992; U.S. Pat. No. 5,216,478 which issued on Jun. 1, 1993 to Kadowaki et al, and Canadian Patent Application 2,165,136 by Pawluczyk et al which was opened to public inspection on Jun. 14, 1997. Each of the above approaches makes use of two diffractive optical components, the first acts as a beam-splitter dividing the incident laser beam into two and the second brings them back together at some convenient point down-stream. Appropriate use of diffractive orders makes this possible with the separation of the fringes remaining constant even for a shift of wavelength. This property has proved extremely useful in the past when using light sources whose wavelengths tended to drift from their nominal settings.
In other systems, such as the one described in U.S. Pat. No. 4,334,779 which issued on Jun. 15, 1982 to Jacques Domey et al, as well as U.S. Pat. 4,948,257 which issued on Aug. 14, 1990 to Kaufmann et al, the initial beam is split using a beam-splitter cube. This, at least partially refractive approach is generally more efficient than the diffractive approach above and assures that there are no extra beams generated which could possibly interfere and upset the integrity of the fringe pattern. The fact that the actual fringe spacing is dependent on wavelength and varies linearly with it, is no longer an insurmountable problem. This is because laser diode sources, stabilized at a given wavelength, can now be obtained for a relatively small penalty in cost. However, the use of beam-splitter cubes or plates, for example those which employ a beam-splitter interface which reflects half the light incident on it, can create stability problems. If the beam incident on the beam-splitter experiences a change in direction by reason of some instability upstream in the system or the beam-splitter itself being subjected to an angular displacement, then this would result in a serious degradation in the accuracy of the instrument. For example, if there is a 4 degree convergence of the two interfering beams and a change in direction of the incident beam on the reflective beam-splitter surface in glass of just 5 access, this would translate to an error of around 0.1% in length measurements.
The prior art length measuring devices are directed to the measurement of surfaces that are without structure and/or are diffuse. They are not capable of measuring surfaces that are structured and at the same time specular. Specular surfaces that are undulating, such as corrugated, wavy or prismatic surfaces, tend to have substantial slope variations over their surface, which result in radiation being deflected from the surface at angles greater than .+-.30.degree.. The prior art devices are unable to collect and detect radiation from such a broad cone and therefore would produce inaccurate measurements.